Category: Drivetrain

Here’s “boost creep”: an uncontrolled increase in manifold pressure, seen here as the light blue “MAP” trace at the center of the figure. This is with a 4-lb spring which in a perfect world limits boost to 4 psi, but the manufacturer notes that actual boost will vary engine to engine. In mine it’s about 7 psi (150kpa), which is fine if it stays there. It holds pretty well until about 5000 rpm, but then the exhaust gas volume becomes too much for it to vent off and the turbine speeds up and increases boost. I let off the gas when it hit 13 psi at 6100 rpm because it was trending upward pretty fast (and because I was already going 84 mph). The right way is to do all this on a dyno – maybe after boost is under control.

Parts were (again) ordered to change to a dual 38mm wastegate setup. I figure that worst case, the two don’t do the job and the header has to be redesigned, and both will still be needed in that case so they aren’t a waste. Somewhat related, I found this funny quote which is a pretty good description of what it’s like to live with a turbo engine, where:

… it’s like having a cokehead pornstar girlfriend. There’s going to be unparalleled excitement and thrills, but a lot of unexplainable downtime and a likely violent ending that leaves you broke and insane…

Back-filled 2011 ; all early posts are under the “Archive” link above.

Disconnected the upper wastegate hose to ensure there’s nothing keeping it from working properly but boost creep persists. I had a talk with a buddy who really knows turbo:

“You increased compression 8:1 to 9:1 (~12% more flow), right?” Yes

“You added cams that flow more air.” Yes

“You switched from a 0.82 AR turbine housing to a 1.06.” Yes

“You’re running E85, which produces a lot more exhaust gas than gasoline.” Yes

“You built an exhaust header with poor flow to the wastegate.” Um, yes

He went on to explain that the 1.06 AR turbine housing is far less restrictive to exhaust flow. That’s a very good thing but means that given the choice, exhaust gas would much rather go through a low-restriction turbine housing instead of making a hard turn into a smallish wastegate. He recommended one or more of the following:

Change the geometry of the header to provide a more direct shot at the wastegate: This is the right way but there’s no room

Add a second 38mm wastegate: There’s room, just

Replace the single 38mm wastegate with a 60mm unit: The “nuclear option.” Builders use these to regulate 1200-hp engines, but much more often, as a Band-Aid to limit boost creep caused by poor wastegate routing. Not an option because chassis tubing blocks the area.

Of the above, adding a second 38mm unit works the best short of starting over. For what it’s worth, two 38mm wastegates have the equivalent valve surface area of a 54mm unit, so it’s not vastly different than going with the 60mm unit.

Well that didn’t last long. Woke up this morning realizing I didn’t need some of the parts ordered last night for the exhaust modification, while a couple other parts had been forgotten. Canceled those parts, then found the wastegates are backordered with an unknown delivery date (at least from this vendor) so everything was canceled. It’s not entirely bad as it gives time to do a few more tests, like disconnecting the upper hose from the wastegate (which can keep the wastegate valve shut if pressurized, increasing boost). It “should be” zero but if it isn’t that would do it, but if it’s zero, that leaves wastegate routing and/or the engine changes (compression, cams, and larger turbine) as causing the wastegate to be unable to regulate a setpoint.

It’s too warm and humid to be doing much in the garage so the time was spent back-filling the 2012 blog.